A radiation detector generally includes a photon detector for detecting photons generated by radiation that is incident on a phosphor such as a scintillator, an amplifier for amplifying signals detected by the photon detector, and an analog-to-digital (A/D) converter for analog-to-digital-converting (A/D-converting) the signals amplified by the amplifier. In this case, an output dynamic range of the A/D converter is maximum in the dynamic ranges of energy due to the radiation acquired by the radiation detector. Causing a radiation detector to effectively detect radiation requires a combined gain of a photon detector and an amplifier serving as input stages of an A/D converter to be close to the dynamic range of the A/D converter.
In a radiation detector of a photon counting system used under a high counting condition such as medical uses, an avalanche photodiode (APD (silicon photo multiplier (SiPM)) operating in the Geiger mode, a photomultiplier tube and the like are used as a photon detector. Output from a photon detector is a high-speed current pulse. Accordingly, a circuit that serves as an interface of a photon detector is required to be wideband. When a photon detector is included in X-ray computed tomography (CT) and the like, a large number of detectors and detecting devices are required to be disposed in a limited space such as a gantry, and the photon detector is required to have low power consumption.
However, conventionally, an operational amplifier having a large gain bandwidth and high power consumption has been used for configuring a circuit that is wideband and causes a band to remain constant even after gain control.